The current telephone network is conceived so that the operations the subscriber needs to carry out when making a telephone call are reduced to a minimum. The subscriber merely has to unhook his receiver in order to obtain a signal tone and then dial the desired number.
However, there is one drawback to this simple set of operations: it is relatively simple for an unauthorized user to connect a telephone set to the telephone line at a point located between the subscriber telephone set and the corresponding telephone exchange. The unauthorized user may then telephone as he pleases by means of this telephone set, the cost of calls being charged to the subscriber of the telephone set.
Devices have already been embodied making it possible to overcome this drawback. Those devices which come closest to the present invention are those which consist of a circuit inserted between the two wires of the line, this circuit being able to assume two states, namely one with high impedance authorizing normal functioning of the line and the other with low impedance prohibiting functioning of the line.
The device described in the patent no. EP-0 126 496 is a device of the above-mentioned type. It includes a NOT-circuit consisting of a first arm constituted by a Zener diode and a resistor, and a second arm constituted by a switch and a low power resistor. When the subscriber unhooks the receiver, means, which continuously detect the condition of the line, shut off the switch, which branches or shunts the line current through the second arm. Dialling, supposedly completed by a telephone set with a dial modulating the all or nothing current, is then rendered ineffective, the exchange not detecting any breaking-off of the current.
The device described in this document also includes means to recognize a code transmitted from the telephone set and, in the event of jamming with a confidential code previously recorded, to put out of service the NOT-arm and thus authorize subsequent dialling.
If this device is satisfactory in certain respects, it nevertheless has the drawback of being limited to dial telephone sets where dialling is effected by pulses. Thus, it currently more frequently does not function on key-operated telephones with audiofrequency dialling.